1. Field of the Invention
This invention relates to communications networks. More particularly, this invention relates to methods and systems for improved utilization of communications networks configured as layer-2 ring networks.
2. Description of the Related Art
The meanings of acronyms and certain terminology used herein are given in Table 1.
TABLE 1ASAutonomous SystemATMAsynchronous Transfer Mode. A networktechnology based on transferring data in cellsor packets of a fixed size.FECForwarding equivalence classHDLCHigh-level Data Link ControlIETFInternet engineering task forceIGPInterior Gateway ProtocolIPInternet protocolLANLocal Area NetworkLDPLabel distribution protocolLLCLogical link congtrolLSALink state advertisementLSPLabel-switched pathLSRLabel-switching routerMACMedia access controlMPLSMulti-protocol label switchingMPLS-TEMPLS traffic engineeringOSIOpen System Interconnection. A networkingframework for implementing protocols.OSPFOpen Shortest path First. A routing protocolOSPF-TEOSPF enhancements used in traffic engineeringRFCRequest for commentsRIPRouting information protocolRPRResilient packet rings - a protocolRSVPResource reservation protocolRSVP-TEAn extension of RSVP, used in trafficengineeringSRPSpatial reuse protocolTETraffic engineeringTLVType-Length-Value. An encoding scheme
Local Area Networks (LAN's) connect computing systems together. LAN's of all types can be connected together using Media Access Control (MAC) bridges, as set forth in the “IEEE Standard for Information Technology, Telecommunications and Information Exchange between Systems, Local and Metropolitan Area Networks, Common Specifications, Part 3: Media Access Control (MAC) Bridges,” published as ANSI/IEEE Standard 802.1D (1998). The 802.1D standard is available via the Internet at the URL standards.ieee.org/catalog/IEEE802.1.html.
Data networks, including LAN's, are commonly conceptualized as a hierarchy of layers according to the Open System Interconnection Model (OSI). OSI defines a networking framework for implementing protocols in seven layers, of which layer-3 (network layer), and layer-2 (data link layer) are relevant to the instant invention.
Implementation of layer-3 requires high level knowledge of the network organization, and access to router tables that indicate where to forward or send data. This layer provides high level switching and routing technologies, and creates logical paths, known as virtual circuits, for transmitting data from node to node. In layer-3, data is transmitted by creating a frame that usually contains source and destination network addresses.
Layer-2 encapsulates the layer-3 frame, adding more detailed data link control information to form a new, larger frame. Layer-2 implements a transmission protocol and handles flow control, frame synchronization, and handles errors arising in the physical layer (layer-1). Layer-2 is divided into two sublayers: a media access control (MAC) sublayer and a logical link control (LLC) sublayer. The MAC sublayer controls how a computer on the network gains access to the data and its permission to transmit the data. The LLC layer controls frame synchronization, flow control and error checking.
HDLC (High-level Data Link Control) is a related term that refers to a group of layer-2 protocols or rules for transmitting data between network points, known as nodes. In HDLC, data is organized into frames and sent across a network to a destination that verifies its successful arrival. The HDLC protocol also manages the flow or pacing at which data is sent.
The Open Shortest Path First (OSPF) protocol is a link-state layer-3 routing protocol used for Internet routing. OSPF is described in detail by Moy in OSPF Version 2, published as Request for Comments (RFC) 2328 of the Internet Engineering Task Force (IETF) Network Working Group (April, 1998), which is incorporated herein by reference. This document is available at www.ietf.org, as are the other IETF RFC and draft documents mentioned below. OSPF is used by a group of Internet Protocol (IP) routers in an Autonomous System (AS) to exchange information regarding the system topology. The term “Autonomous System” denotes a group of routers exchanging routing information via a common routing protocol. Each OSPF router maintains an identical topology database, with exceptions as noted below. Based on this database, the routers calculate their routing tables by constructing a shortest-path tree to each of the other routers.
Each individual piece of the topology database maintained by the OSPF routers describes the “local state” of a particular router in the Autonomous System. This local state includes information such as the router's usable interfaces and reachable neighbors. The routers distribute their local state information by transmitting a link state advertisement (LSA). Packets containing link state advertisements are flooded throughout the routing domain. The other routers receive these packets and use the LSA information to build and update their databases.
OSPF allows collections of contiguous networks and hosts to be grouped together to form an OSPF area. An OSPF area includes routers having interfaces to any one of the grouped networks. Each area runs a separate copy of the basic link-state routing algorithm. The topology of an OSPF area is invisible from outside of the area. Conversely, routers internal to a given area does not know the detailed topology external to the area. This isolation of knowledge results in a marked reduction in routing traffic, as compared to treating the entire Autonomous System as a single link-state domain. A router in an Autonomous System has a separate topological database for each area to which it is connected. Routers connected to multiple areas are called area border routers. However, routers belonging to the same area have, for that area, identical area topological databases.
An OSPF LSA database allows a layer-3 aware network element, such as a router, to build its routing table by running the well-known SPF algorithm. The element then routes IP packets based on the actual routing table and on the destination IP address in the IP packet header. A cost is associated with the output side of each router interface, and is used by the router in choosing the least costly path for the packets. This cost is configurable by the system administrator. The lower the cost, the more likely the interface is to be used to forward data traffic. For the purposes of cost calculation and routing, OSPF recognizes two types of networks (which may be organized as IP networks, subnets or supernets): point-to-point networks, which connect a single pair of routers; and multi-access networks, supporting two or more attached routers. Each pair of routers on a multi-access network is assumed to be able to intercommunicate directly. An Ethernet is an example of a multi-access network. Each multi-access network includes a “designated router,” which is responsible for flooding LSA's over the network, as well as certain other protocol functions. Further details concerning network cost calculation and routing are disclosed in application Ser. No. 10/211,066, (Publication No. 20030103449), which is commonly assigned herewith, and herein incorporated by reference.
Multi-access layer-2 networks may be configured internally as rings. The leading bi-directional protocol for layer-2 high-speed packet rings is the Resilient Packet Rings (RPR) protocol, which is in the process of being defined as IEEE standard 802.17. Network-layer-routing over RPR is described, for example, by Jogalekar et al., in IP over Resilient Packet Rings (Internet Draft draft-jogalekar-iporpr-00), and by Herrera et al., in A Framework for IP over Packet Transport Rings (Internet Draft draft-ietf-ipoptr-framework-00). A proposed solution for media access control (MAC protocol layer-2) in bi-directional ring networks is the Spatial Reuse Protocol (SRP), which is described by Tsiang et al., in the IETF document RFC-2892, entitled The Cisco SRP MAC Layer Protocol. Using protocols such as these, each node in a ring network can communicate directly with all other nodes through either an inner or an outer ring, using the appropriate Media Access Control (MAC) addresses of the nodes. The terms “inner” and “outer” are used arbitrarily herein to distinguish the different ring traffic directions. These terms have no physical meaning with respect to the actual configuration of the network.
Multiprotocol Label Switching (MPLS) is gaining popularity as a method for efficient transportation of data packets over connectionless networks, such as Internet Protocol (IP) networks. MPLS is described in detail by Rosen et al., in Request for Comments (RFC) 3031 of the Internet Engineering Task Force (IETF), entitled “Multiprotocol Label Switching Architecture” (January, 2001). In conventional IP routing, each router along the path of a packet sent through the network analyzes the packet header and independently chooses the next hop for the packet by running a routing algorithm. In MPLS, however, each packet is assigned to a Forwarding Equivalence Class (FEC) when it enters the network, depending on its destination address. The packet receives a short, fixed-length label identifying the FEC to which it belongs. All packets in a given FEC are passed through the network over the same path by label-switching routers (LSR's). Unlike IP routers, LSR's simply use the packet label as an index to a look-up table, which specifies the next hop on the path for each FEC and the label that the LSR should attach to the packet for the next hop.
Since the flow of packets along a label-switched path (LSP) under MPLS is completely specified by the label applied at the ingress node of the path, a LSP can be treated as a tunnel through the network. Such tunnels are particularly useful in network traffic engineering, as well as communication security. MPLS tunnels are established by “binding” a particular label, which is assigned at the ingress node to the network, to a particular FEC.
Currently, layer-3 routing protocols, such as RIP and OSPF, are unaware of the topology of layer-2 RPR networks with which they must interact. A routing table allows the router to forward packets from source to destination via the most suitable path, i.e., lowest cost, minimum number of hops. The routing table is updated via the routing protocol, which dynamically discovers currently available paths. The routing table may also be updated via static routes, or can be built using a local interface configuration, which is updated by the network administrator. However, the RPR ingress and egress nodes chosen in the operation of automatic routing protocols do not take into account the internal links within the RPR ring, and may therefore cause load imbalances within the RPR subnet, which generally results in suboptimum performance of the larger network.